So called “negative” storage yarn feeders are known, which are provided with a stationary drum on which a motorized swivel arm winds a plurality of yarn loops forming a stock. Upon request from a downstream textile machine, e.g., a conventional circular/rectilinear knitting machine, the yarn loops are unwound from the drum, then the yarn passes through a controlled yarn-braking device which modulates its tension in order to maintain it substantially constant on a predetermined value, and finally is fed to the machine.
With yarn-braking devices such as the one disclosed in EP-B-622 485, the yarn is pressed between a stationary plate and a movable braking member, which is also shaped as a plate and is driven by a linear motor. With yarn-braking devices such as the one disclosed in EP-B-1 059 375, the unwinding yarn is pressed between the delivery edge of the drum and a hollow, frustoconical braking member connected to a motor. In any cases, the motor driving the braking member is controlled by a control unit provided with a closed control loop, which receives a measured tension signal from a tension sensor arranged downstream of the feeder, compares it with a reference tension representing a desired tension, and modulates the braking action in such a way as to minimize the difference between the measured tension and the reference tension.
The conventional control systems are designed to compensate for slow changes in the tension, which may be generated, e.g., by the wear of the braking members, and are tuned in such a way as to be substantially indifferent to any small, sudden variations of tension, which may be generated, e.g., by a node or by a portion of yarn having an irregular thickness. To this purpose, the control loop is set to a very low frequency band, even below 0.1 Hz.
It is also known from EP2031106 to enable the control loop only when the yarn consumption speed exceeds a predetermined threshold value, in order to prevent the control system from operating improperly during particular operative conditions, e.g., at the start of the weaving process, when the knitting machine is not running yet, or during the insertion step, when the yarn is not running.
Although the known control systems are satisfactory for those applications in which the reference tension is rarely, or even never, changed during processing, they are not suitable to those applications in which the reference tensions are frequently changed at short time intervals during processing, e.g., in order to produce meshes having particular fancy patterns. In fact, due to the very low frequency band, several seconds are required to align the yarn tension to the new value, but this delay is not acceptable with the above processes.
Therefore, nowadays these processes can be only performed on expensive knitting machines which allow the stitch cams to be adjusted, while they cannot be carried out on less sophisticated, older machines which do not have this function.